1. Field of the Invention
The present invention relates to tubular elements, such as control rods and rod control cluster assemblies, for use in high-pressure and high-temperature water of nuclear reactors, such as pressurized water nuclear reactors. In particular, the present invention relates to a system and method for treating these tubular elements to improve their wear resistance and corrosion resistance.
2. Background
In a nuclear power plant, the fuel assemblies of the nuclear reactor core employ rod control cluster assemblies (“RCCAs”) to control reactivity. The power produced by the nuclear reactor is generally controlled by raising (e.g., retracting) or lowering (e.g., inserting) the RCCAs within the reactor core. The change in reactor power output required to accommodate a change in the demand for output from the power plant is commonly referred to as load follow.
The RCCAs consist of a plurality of neutron-absorbing control rods fastened at their top ends to a common hub or spider assembly. The body of each of the control rods generally includes a stainless steel tube which encapsulates a neutron-absorbing material. Each of the control rods is slid within a tubular guide thimble tube of the fuel assembly. A control drive mechanism near the top of the spider assembly operates to control the movement of the control rod within the guide thimble tube. In this manner, the controlled insertion and extraction of the control rods generally controls the amount of power produced by the nuclear reactor.
As a result of the operation of the RCCAs in the nuclear reactor, they can be subjected to frictional and vibrational wear. The sliding movement of the control rods being inserted and extracted relative to the guide thimble tubes produces friction. This friction can cause the control rods to wear. Further, the impacting and rubbing of the control rods with the guide thimble tubes as a result of flow induced vibration during reactor operation can also cause the control rods to wear. Furthermore, cooling water in the Spent Fuel Pool (“SFP”) typically contains boron (e.g., 2100-2700 ppm boron) and contact of the cooling water with the RCCAs can cause corrosion of the control rods and pollution of the fluid systems. The frequency and the amplitude of the movements of certain control rods, particularly, when the reactor is used in the load follow mode, are such that it may be necessary to systematically replace a certain number of RCCAs on each core reloading.
It is known in the art to deposit a protective coating on the outer surface of such tubular elements subject to friction to reduce the wear thereof. For example, electrolytic platings of hard chromium and chemical platings of nickel have been produced on control rods and RCCAs. There have been disadvantages experienced with these platings, such as, electrolytic chromium platings are generally fragile and chemically deposited nickel may contaminate the primary circuit of the reactor. Furthermore, coatings containing chromium carbide to which a nickel-chromium bonding alloy is added have demonstrated poor behavior under irradiation.
It is also known in the art to deposit a protective coating by carbon nitriding the outer surface of a tubular element, such as a control rod tube or guide thimble tube, for a nuclear reactor. Nitridation is carried out ionically. The tubular element is subjected to a plasma created by electrical discharge in a low pressure atmosphere containing nitrogen and hydrogen. Active nitrogen ions are implanted at a sufficiently high temperature to cause in-depth diffusion of the ions. Typically, the durations and temperatures of treatment are such that the processed coating layer does not exceed a maximum depth or thickness. In practice, the nitridation process has been conducted at a temperature of from about 500° C. to 550° C. for a period of varying hours. Further, in practice, it has been observed that nitridation, in accordance with these known temperatures, can result in the formation of chromium-nitride and depletion of chromium from the surface of the coating. As a result, the tubular elements can exhibit poor corrosion resistance in the reactor and spent fuel pool environments.
It is desired to develop an improved system and process for the nitridation of tubular elements, such as control rods and RCCAs, to avoid chromium nitride formation while achieving a treated surface having a maximum thickness which is effective in resisting frictional wear and/or corrosion.